The invention relates to a method of forming ordered mesoporous silicon carbide nanocomposite materials. The ordered mesoporous silicon carbide nanocomposites are made using an aqueous precursor composition that comprises at least one surfactant, an oil, a carbon precursor, and a silica precursor. The inventive method affords control over the inorganic phases during synthesis, as well as the mesoporous structure, size, surface area and macroscale morphology of the resulting nanocomposite materials.
Non-oxide ceramics have been the subject of investigatory research because they possess an advantageous array of electrical, mechanical and other functional properties. Silicon carbide (SiC), for example, is a semiconducting material with high temperature mechanical stability, high hardness, and excellent heat conductivity. Due to its chemical inertness and ability to tolerate harsh environments, it has been considered for use as a catalyst support. However, commercially available silicon carbide has a low specific surface area, which makes it unsuitable for catalytic applications. As a result, new methods are being developed for preparing high surface area silicon carbide.
One technique for preparing high surface area silicon carbide involves the infiltration of high surface area silica with a SiC precursor followed by the removal of the silica template by etching with HF. Examples of this method involve the chemical vapor or liquid infiltration of the silicon carbide precursor, or the infiltration of nano-sized silica spheres into the template. Such methods can be used to form a high surface area silicon carbide having a disordered structure.
An additional technique for preparing high surface area silicon carbide involves the infiltration of a silica precursor into a porous carbon substrate. By tailoring the C/SiO2 molar ratio, porous, disordered, crystalline SiC nanoparticles and nanofibers can be formed.
In view of the foregoing, there is an interest in preparing ordered, porous silicon carbide materials comprising an open (high surface area) framework. In addition to catalysis, such materials can be used in fuel cells and solar cells, and in applications comprising adsorption and/or separation chemistry. Because versatility in both structure and form of these materials is highly desirable, it would be advantageous to provide an improved, economical synthesis with enhanced processing capability.
In accordance with the present invention, Applicants have unexpectedly determined that ordered mesoporous silicon carbide materials can be prepared by forming an aqueous precursor mixture comprising carbon and silicon precursors, a non-ionic surfactant, and an oil (e.g., water-immiscible liquid), drying and cross-linking the precursor mixture to form an intermediate product, and heating the cross-linked intermediate. The heating process drives three reactions: 1) carbonization of the carbon precursor, 2) condensation of the silica precursor, and 3) carbothermal reduction of the precursors to form silicon carbide.
After preparing the precursor mixture, but before cross-linking of the carbon and silicon precursors, the surfactant self-assembles to form a template for the precursors, which defines a mesoscale liquid crystal phase that, upon heating and removal of the surfactant, forms a silicon carbide composite material that comprises ordered domains of mesoscale porosity.
Ordered mesoporous silicon carbide nanocomposite materials comprise a three-dimensionally ordered and interconnected array of pores that range in size from about 2 to 50 nm. These materials may exhibit BET specific surface areas as high as about 2200 m2/g, and typically exhibit excellent thermal stability in inert atmospheres, and strong resistance to attack by acids and bases.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.